1. Field of the Invention
The present invention relates to a seismic data acquisition system using acquisition stations set on the bottom of a water mass.
2. Description of the Prior Art
Activity in the field of seismic exploration of underground zones lying deep under water ranging between about 300 m and 1500 m (referred to as deep offshore) is developing rapidly. The average size of the oil fields that are going to be developed in future increases considerably with the water depth. Since the start-up costs, the production costs and the associated risks are all the higher as depth increases, it is all the more important to optimize the production. This requires new exploration and production technologies allowing reduction of uncertainties and more rapidly producing at an acceptable cost.
Seismic methods are developing constantly. However, despite improved results, their cost is still considered to be too high to be applied systematically. Multicomponent seismic prospecting (3C/4C) and seismic prospecting referred to as repetitive (4D) are clearly identified currently as technologies with the highest expansion potential and which will join together to such an extent that it will be possible to manage underground reservoirs by using the additional data available and to combine them with the data obtained from the instruments in the well. The challenge consists in putting on the market a seismic exploration system with pickups located on the water bottom at a reasonable cost, while taking account of the profit provided by such systems in terms of information abundance and logistic advantages. The most suitable exploration system must meet the requirements of seismic prospecting for reservoir characterization as well as reservoir monitoring during development, notably under deep offshore conditions, while being reliable, effective and economical.
OBC type seismic prospecting methods using a seismic cable or streamer laid on the sea bottom are well-known. Seismic receivers such as geophones mounted on U-joints to provide correct orientation or hydrophones are arranged all along this part and are coupled with the bottom. The receivers are connected by lines internal to the streamer to acquisition equipment on a boat at the surface. Several cables can be laid in parallel, thus forming a 3D seismic device. Such streamers are commonly used down to depths of about 500 m and even, in some cases, of more than 1000 m. The streamers are towed immersed within the framework of seismic prospecting operations or permanently installed within the context of long-term monitoring (4D seismic monitoring). A second boat is used to move the seismic source. This technique is especially used in obstructed zones and in transition zones. Unless the cables are buried in the sediments, coupling of the pickups with the sea bottom is not optimal and, despite a large number of pickups, the seismic data acquired are of average quality.
Devices of this type are for example described in U.S. Pat. No. 4,870,625 and U.S. Pat. Nos. WO-99/23,510, 98/07,050 and U.S. Pat. No. 97/13,167.
It is also well-known for oceanographic research, notably for surveys of the structure and the seismicity of the continental margin, to lower down to the sea bottom (OBS type) acquisition stations consisting each of a sealed box containing seismic receivers, hydrophones and geophones, etc., and the associated electronic equipment, allowing continuous recording of low-frequency seismic signals and storage of the data in a mass memory. Coupling of the pickups in the sedimentation is satisfactory. Recovery of the acquired data occurs after pulling the equipment from the bottom to the surface. A launching device activated by acoustic control from the boat at the surface allows disconnection of a ballast, then pulling of the acquisition equipment to the surface, and signaling devices such as a flag and a warning light allow to locate it at sea. After each use, a station is reconditioned prior to a new immersion. Most of these stations work at depths that can reach 6000 m. The number of stations used for a scientific mission is relatively small and the distance between the stations can range from several hundred meters to several hundred kilometers. The recording time, which can range from one week to several months, is conditioned by the data storage capacity and by the range of the supply battery.
Systems of this type are for example described in U.S. Pat. Nos. 4,422,164, 4,692,906 and U.S. Pat. No. 5,189,642.
It is also well-known to position on the sea bottom seismic data acquisition units so as to improve coupling of the pickups with the underlying formation.
This positioning can be performed using a subsea robot or by launching from the surface acquisition units that are sufficiently streamlined to directly penetrate the bottom under the effect of gravity. These seismic signal acquisition units include a streamlined part or boom provided with a housing for at least one seismic receiver (a 3C geophone and a hydrophone for example), inclinometers for measuring their orientation and a compass and a seismic data collection module, as well as means of positioning each acquisition unit at the bottom (acoustic telemetry), and surface equipment for recovery of the seismic data collected. Recovery of these units at the end of an operation is also performed by means of the robot. A system of this type is made out to work at water depths up to 1500 m.
Acquisition systems of this type are for example described in French patent application 00/16,536 and in French patent 2,738,642 and U.S. Pat. No. 4,134,097.
French patent 2,774,775 filed by the assignee also describes a method for seismic prospecting or monitoring of an offshore reservoir wherein one or more seismic emission units comprising one or more seismic sources associated with a power supply are lowered to the sea bottom, each one of these units being connected to the surface by a multifunction cable. Seismic receivers can also be coupled with the surface of the bottom.
The underwater acquisition devices, whether mobile or coupled with the water bottom are generally associated with acoustic positioning devices. The relative position of the devices in relation to several surface buoys provided with satellite location equipment (GPS positioning system) is located by acoustic telemetry. Locating devices combining acoustic telemetry and satellite positioning are for example described in U.S. Pat. No. 5,119,341 and U.S. Pat. No. 5,579,285.